Structural Characterization and Emulsifying Properties of Highly Soluble Macadamia–Soybean Protein Composites Fabricated by Alkaline-Thermal Treatment

The complementarity of plant proteins from various sources could achieve higher nutritional value to satisfy the requirement of replacing animal proteins. Therefore, it is very important to seek efficient and convenient approaches to fabricate highly soluble protein composites. In this study, macadamia protein-soybean protein (SP-MP') composites were fabricated by alkaline-thermal treating at different ratios of 1:0.5, 1:1, and 1:2; then, the nitrogen solubility index, particle characteristics, and structure and emulsifying properties of SP-MP' composites were investigated. The nitrogen solubility indexes of SP-MP' composites were higher than 80%, and less small insoluble aggregates were observed by scanning electron microscopy. SP-MP' composites exhibited high ζ-potential values, which were higher than -50 mV. Sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE) analysis found that both subunits of individually alkaline-thermal-treated macadamia protein (MP') and soybean protein (SP') were presented in SP-MP' composites. The results of fluorescence, sulfhydryl group, and secondary structure illustrated that the SP interacted with MP to form SP-MP' composites by the co-folding of proteins during neutralization. Compared to the individual proteins, SP-MP' composites exhibited stronger emulsification ability and stability indexes (EAI and ESI) as the proportion of MP increased, and the EAI and ESI of SP-MP_1:2' were 21.53 m²/g and 146.7%, respectively. Meanwhile, emulsions prepared by SP-MP' composites displayed more uniform oil droplet distributions. The findings suggested that highly soluble SP-MP' composites with stronger emulsification abilities were successfully fabricated, which have great potential as ingredients to manufacture nutritional plant protein beverages.